Swallowtails key to azalea pollination

A Spicebush Swallowtail, Papilio troilus, pulls nectar from the blossom of a Pinxter-flower azalea, Rhododendron periclymenoides. A rival flutters in from the left, and it displaced the other butterfly a second later.

I made this image back on May 15 in Shawnee State Forest, Scioto County, Ohio. The azaleas were in full bloom, and offered irresistible photo subjects. It didn't take long to see that swallowtail butterflies were drawn to the blossoms in large numbers, and soon those became the target of my lens.

Pinxter-flower is a rare plant in Ohio, with an official designation of threatened. Just to roil the botanical waters a bit, there is a very similar species, the so-called Roseshell Azalea, Rhododendron prinophyllum. The latter differs in its shorter stamens and pubescent undersurfaces of the leaves. At one time they were lumped (rightly so?) as varieties under one species, Rhododendron nudiflorum.

The blurry taxonomy of this beautiful azalea is not the point here, anyway. Its pollination is. A few weeks after I shot this photo, and observed numerous swallowtails pillaging the pinxter-flowers, an interesting paper came out from a researcher at North Carolina State University. Biologist Mary Jane Epps, along with colleagues Suzanne Allison and Lorne Wolf, published a study entitled Reproduction in flame azalea (Rhododendron calendulaceum Ericaceae): A rare case of insect wing pollination.

In this paper, Epps et al show how the wings of swallowtail butterflies can span the large gap between the anthers (pollen structures) and stigma (female flower part) of azalea flowers. Close examination of the wings of swallowtails showed that pollen granules did indeed adhere to the butterflies' scaly wings, and if swallowtails were excluded from visiting azalea plants, pollination rates plummeted. Smaller pollinating insects just cannot bridge the distance from anther to stigma, and transfer pollen from anther to stigma. The large fluttering wings of swallowtails are perfect for the job.

While the target of Epps' research was Flame Azalea, it should be noted that that species is very similar structurally to Pinxter-flower and it would seem likely that the same pollinator requirements come into play. As a footnote to plants in the genus Rhododendron, the species with deciduous leaves are the azaleas; those with leathery evergreen leaves are generally called rhododendrons.

For those into photography, I made the image using my bird rig: Canon 7D Mark II mounted to Canon's 500mm f/4 II lens, on a tripod. Shooting the butterflies posed a topographical challenge. The best, most floriferous azaleas were on very steep slopes and it was nearly impossible to ascend to them and balance one's self on the loose soil near the plants. So I discarded the idea of using close-range macro gear and just set up the big rig on a flat stable surface by the road below. Worked like a charm, and I was able to get better images than I would have with the usual macro setup. This would only work well on the largest butterflies such as swallowtails. I'll stick with macro gear for lesser butterflies.